Selective catalytic reduction (SCR) provides a method for removing nitrogen oxides (NOx) emissions from fossil fuel powered systems for engines, factories, and power plants. During SCR, a catalyst facilitates a reaction between exhaust ammonia and NOx to produce water and nitrogen gas, thereby removing NOx from the exhaust gas. Generally, the exhaust gas ammonia is mixed with an exhaust gas stream upstream of an SCR catalyst. The ammonia may be supplied directly into the exhaust gas stream or provided as urea, which can be converted to ammonia under appropriate conditions.
To optimize NOx removal from the exhaust gas stream, while preventing release of potentially noxious ammonia or urea, the NOx and ammonia are generally supplied at approximately one-to-one ratios. However, it may be difficult to match the quantities of ammonia and NOx flowing into the SCR system. Further, even when the ratios are perfectly matched, some of the NOx may not be reduced due to inadequate mixing, short exhaust gas transit times, or sub-optimal catalyst operating conditions. Therefore, improved SCR systems that provide better NOx removal are needed.
One system for controlling NOx emissions is disclosed in patent publication WO 2004/058642, which was filed by Valentine on Dec. 17, 2003 (hereinafter the '642 publication). The '642 publication provides a multi-stage NOx reduction system. The system may include multiple catalysts effective at different temperature ranges and having injectors associated with each.
While the system of the '642 publication may provide suitable NOx-emissions control for some applications, the system of the '642 publication may have several drawbacks. For example, the system of the '642 publication may require on-board storage of reductants, which may require significant space and may be hazardous. Further, the system of the '642 publication may use more reductant than is needed, thereby wasting chemicals and potentially releasing the reductant into the environment. Further, the system of the '642 publication may not provide optimum NOx-emissions control for some applications.
The present disclosure is directed at overcoming one or more of the shortcomings of the prior art NOx-emissions control systems.